1. Field of the Invention
The invention relates to rotary drag-type drill bits for drilling in subsurface formations. In particular, the invention is a rotary drag-type drill bit with wearable pads formed of an impregnated diamond/matrix material.
2. Description of Related Art
Diamond impregnated bits may be generally described as being of the kind comprising a bit body having a leading surface comprising a plurality of pads separated by channels for drilling fluid, at least some of the pads each having an abrasive surface including particles of superhard material. The particles of superhard material may be natural or synthetic diamonds, or small bodies of polycrystalline diamond, set in the outer surface of a pad, or the pad may include an outer layer impregnated with superhard particles which, again, may be natural or synthetic diamonds or particles of polycrystalline diamond. It is also possible to combine the two arrangements. Such bits are particularly suitable for drilling through harder subsurface formations.
Hitherto, it has been the usual practice for all parts of the abrasive surfaces of the pads to lie on the same profile. That is to say, at each longitudinal position on the bit, all points on the surfaces of the pads are at the same radius with respect to the central longitudinal axis of the bit. Accordingly, during drilling all of the abrasive surfaces of the bit act on the formation being drilled simultaneously.
In many applications, a lighter set bit (i.e., a bit having fewer superhard particles disposed over the surface of the bit) would be preferable when drilling the upper part of the formation, but a heavier set bit is actually used since such a bit will be required in order to drill through the harder formations likely to be met deeper in the borehole. As a result of having to use the heavy set bit in the softer formations near the surface, the rate of penetration (ROP) of the bit may be lower than that could be achieved by using a lighter set bit.
The present invention therefore sets out to provide a rotary drill bit of the above-mentioned type where the same bit can act as a lighter set bit during initial drilling but can act as a more heavily set bit as drilling proceeds.